1. Field of the Invention
The present invention relates to integrated circuit manufacturing, and more particularly to forming insulated-gate field-effect transistors with densely-packed gate, source and drain contacts.
2. Description of Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in order to modulate the longitudinal conductance of the channel.
In typical IGFET processing, the source and drain are formed by introducing dopants of a second conductivity type (P or N) into the semiconductor substrate of a first conductivity type (N or P) using a patterned gate as a mask. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, the polysilicon is anisotropically etched to provide a gate which provides a mask during formation of the source and drain by ion implantation, and then the implanted dopant is activated using a high-temperature anneal that would otherwise melt the aluminum.
As IGFET dimensions are reduced and the supply voltage remains constant (e.g., 3V), the electric field in the channel near the drain tends to increase. If the electric field becomes strong enough, it can give rise to so-called hot-carrier effects. For instance, hot electrons can overcome the potential energy barrier between the substrate and the gate insulator thereby causing carriers to become injected into the gate insulator. Trapped charge in the gate insulator due to injected hot carriers accumulates over time and can lead to a permanent change in the threshold voltage of the device.
A number of techniques have been utilized to reduce hot carrier effects. One such technique is a lightly doped drain (LDD). The LDD reduces hot carrier effects by reducing the maximum lateral electric field. The drain is typically formed by two ion implants. A light implant is self-aligned to the sidewalls of the gate, and a heavy implant is self-aligned to spacers adjacent to the sidewalls of the gate. The spacers are typically oxides or nitrides. The purpose of the lighter first dose is to form a lightly doped region of the drain (or LDD) at the edge near the channel. The second heavier dose forms a low resistivity heavily doped region of the drain, which is subsequently merged with the lightly doped region. Since the heavily doped region is farther away from the channel than a conventional drain structure, the depth of the heavily doped region can be made somewhat greater without adversely affecting the device characteristics.
Since it is very important to reduce the series resistance of the gate, source and drain for submicron devices, several techniques have been developed to improve contact resistance. One such approach is to use a refractory metal silicide to contact these regions. With this approach, a thin layer of refractory metal is deposited over the structure, and then heat is applied to form a silicide wherever the refractory metal is adjacent to silicon (including single crystal silicon and polysilicon). Thereafter, an etch is applied that removes unreacted refractory metal over the spacers to prevent bridging silicide contacts for the gate, source and drain. Various suicides, including PtSi, MoSi.sub.2, CoSi.sub.2 and TiSi.sub.2 have been used for this purpose. An advantage to this approach is that the silicide contacts for the gate, source and drain are formed simultaneously and are self-aligned by the spacers. This self-aligned silicide is sometimes referred to as "salicide."
After the silicide contacts are formed, typically an oxide layer is formed over the device, contact windows are etched in the oxide layer to expose the silicide contacts, a blanket layer of metallization is deposited over the oxide layer and into the contact windows to provide interconnect metallization, selected regions of the interconnect metallization are removed, and then a passivation layer is deposited over the substrate.
A drawback to conventional silicide contact formation is that typically the spacers are essential to prevent bridging the silicide contacts, and the spacers occupy valuable chip area. Although disposable spacers can be employed and subsequently removed, the additional chip area is still required. Another drawback is that the channel length is often limited by the minimum resolution of the photolithographic system, since a substantial overlap between the gate and the source/drain may create unwanted capacitive effects. These drawbacks tend to constrain efforts towards further miniaturizing the device.
Accordingly, a need exists for a method of fabricating an IGFET that forms densely-packed contacts for the gate, source and drain in order to provide further reductions in device size.